Evolution of cerebral vesicles and their sensory organs in an ascidian larva

The ascidian larval nervous system consists of the brain (comprising the vi sceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ci liary photoreceptors, are contained in the sensory vesicle. A third presump tive sensory organ was sometimes found in an 'auxiliary' ganglionic vesicle . The development and morphology of the sensory and auxiliary ganglionic ve sicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presum ptive mechano-receptor cells (S1 cells), send ciliary and microvillar protr usions to contact the pigment cup. A second group of distinctive cells (S2) , slightly dorsal to the S1 cells, have characteristic microvillar extensio ns, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnan t of a primitive ocellus. In the ganglionic vesicle some cells contain modi fied cilia and microvillar extensions, which resemble the photoreceptor end ings of the photolith. Our results are discussed in the light of two possib le scenarios regarding the evolution of the nervous system of protochordate s.